Apache Friends Support Forum

<!--#include file="header.html"--> was working until a recent update in Windows 10 Pro. It now fails. I can only rename and existing html file to a php file and change the include to <?php include "header.html"; ?>.
Is this a problem with windows or XAMPP configuration?

Its a server configuration. The include Syntax is called "Server Sides Include" (SSI) and is implemented in Apache. I dont know, if Xampp supports SSI and how, but formerly you had to name your files "*.shtml" instead of "*.html" in order to activate the SSI Interpreter. Maybe this is still supported that way in Xampp. Rename to index.shtml (also try index.shtm) and try again.